To fix it, follow the steps below:
1 - In your Login Application open the function remember_me_validate on the menu Programming > PHP Methods
2 - Inside the function remember_me_validate look for the line:
$code .= $chars{mt_rand(0, $max)};
3 - Replace the curly brackets { } in the variable $chars{mt_rand(0, $max)} by square brackets [ ].
The code will be similar to this:
$code .= $chars[mt_rand(0, $max)];
4 - Now save the changes and generate the source code of the application.